Radial anisotropic cylinder type ferrite magnets and their manufacturing methods and motors

ABSTRACT

A high performance radial anisotropic cylinder-shape ferrite magnet for use in a motor to lower its noise and enable miniaturization thereof is formed of an integrated one-piece body formed of sintered Sr and/or Ba ferrite powders and has an axially-extending slit that is created before the body is sintered to reduce internal stress due to shrinkage. The slit can extend in parallel to both the axial and radial directions of the body, diagonally to both the axial and radial directions, or diagonally to the radial direction and parallel to the axial direction. The slit can be filled with a resin material.

TECHNICAL FIELD

This invention relates to Sr ferrite and Ba ferrite compositions thatare used in motors, etc., and in radial anisotropic cylinder typeferrite magnets compositions. This invention further concerns highperformance radial anisotropic cylinder type ferrite magnets and theirmanufacturing methods to motors which do not require use of the usualsegment magnets.

BACKGROUND ART

Conventional constructions of motors in which ferrite magnets arepositioned in stators and rotors are shown in FIGS. 8, 9 and 10.

That is to say, the construction in FIG. 8 is an example of a motorutilizing a cylindrical ferrite magnet 1 on the stator side. The magnetis fastened to the inner circumference of the cylindrical yoke 2, and arotor (not shown here) is placed in space 3 in the inner circumferenceside of the magnet. Usually, for a motor with such construction using astrong magnetic radial anisotropic ferrite magnet is difficult for thereasons explained later, and an isotropic cylinder type ferrite magnet 1is utilized. As such, the motor has a relatively low output.

The construction shown in FIG. 9 is an example of a motor using a pairof segment magnets, 1a and 1b on the stator side. Each magnet isfastened to the inner circumference of the yoke 2, and a rotor (notshown) is positioned in the opposing space 3 on the inner circumferenceside where the magnetic pole system. Other motors use multiple segmentmagnets according to the number of magnetic poles. In such segmentmagnets, since it is possible to use a strong magnetic radialanisotropic ferrite magnet, they are used in relatively high outputmotors.

The construction shown in FIG. 10 is an example of a motor wherein apair of segment magnets 1c and 1d are used on the rotor side. Eachmagnet is fastened to the outer circumference of the magnet support 5and positioned within a stator (not shown) of the specified shape toconfigure a motor. Segment magnets 1c and 1d are ferrite magnets asshown in FIG. 9, and since they are strong magnetic radial anisotropicferrite magnets, the motor produced has a relatively high output.

However, recently even for high output motors, from the point of view ofsimplification assembly (making the assembly operation more efficient)and preventing cogging, it has been sought to find a motor constructionwhich positions radial anisotropic cylinder type ferrite magnets whichhave equivalent or better magnetic properties than the above statedradial anisotropic ferrite magnets.

As a method of manufacturing such radial anisotropic cylinder typeferrite magnets, raw material powder such as Sr ferrite pulverizedpowder and Ba ferrite pulverized powder with an average particle size ofless than 2 micro meter are molded into a cylindrical form using the drymethod under a magnetic field and sintered. Due to the shrinkage factorsfor the circumferential and radial direction differs in sintering,internal stress accumulates to cause easy cracking, preventing theimplementation of this method.

To prevent cracking in the sintering process, it has been proposed touse a mixture of 50˜80 wt % Sr ferrite pulverized powder with an averageparticle size less than 2 micro meter and 50˜20 wt % Ba ferriteisotropic granulated powder with a particle size of 14˜200 mesh in thepresence of a magnetic field end to mold it using the dry method (PatentBulletin Heisei 1-48643).

However, the magnetic characteristic of radial anisotropic cylindershape ferrite magnets industrially sintered utilizing this method haveupper limits of Br=3.4 kG, _(B) H_(C) =2.9 kG, and (BH)max=2.6 MGOe, andthese are not enough to satisfy recent high performance demands.

That is to say, without the availability of radial anisotropic cylindershape ferrite magnets with strong magnetic properties, which areindispensable to realization of sought after high power out-put motors,it is difficult to satisfy this demand.

As explained shove, for example, one body high performance radialanisotropic cylinder shape ferrite magnets consisting of Sr ferrite werehard to manufacture, so that segment magnets which are generally lesssusceptible to cracking were manufactured from these compositions, andas shown in FIG. 9, a pair were positioned at opposing ends, or thesemagnets were assembled into cylinder shape.

Therefore, the utilization of multiple segment magnets not onlycomplicated the assembly processes, but as shown in FIG. 9 when a pairwere positioned at opposing ends, a large space is created betweenmagnets in circumferential directions. Also, when magnets were assembledin a cylindrical shape, many patches (connecting parts) were unavoidablymade in the circumferential directions; thus, creating the problem ofcoggings in motors.

Particularly when these motors were used as wiper motors and fan motors,etc., noise associated with coggings was generated; and it was necessaryto consider the environmental impact for those who work close to suchmotors.

As the other manufacturing method for radial anisotropic cylinder shapeferrite magnets for motors, a special molding die has been proposed(Patent Bulleten Heisei 4-19684). That is to say, in order to avoidcracking when sintering, 2 or 3 pieces of shape edged protrusions areplaced radially to the rod of extrusion molding die or the mandrel ofthe rolling device. When the cylindrical molding is slid toward the axisto be released, inside the inner circumference the cross sectional Vshaped 2 or 3 cuts which are extended axially are formed; and whensintered, and particularly in the cooling process, the internal stressis all focused on these cuts to prevent micro crackings in other partsof the molding.

However, the above stated method prevents complex cracking, and to makerepair by adhesives easy, for example, to obtain above stated segmentmagnets by making cuts and crack magnets at predetermined places byfocussing the internal stress. Therefore, this is not the method toobtain a uniform cylindrical shape ferrite magnet, and it does not solveproblems of segment magnets and their assembly.

This invention concerns the development of the high performance radialanisotropic cylinder shape ferrite magnets, and the objective is to beable to utilize the powder composition that has the strong magneticproperties; and to provide the high performance radial anisotropicferrite magnets in one uniform body without having to assemble segmentmagnets and their manufacturing method. Also, this invention eliminatesthe problems mentioned above, it aims to provide motors that can makethe assembly more efficient, and lower noise and the miniaturizationpossible.

SUMMARY OF INVENTION

Radial anisotropic cylinder shape ferrite magnets which consists thisinvented motor, by cutting the ferrite powder which contains Sr and/orBa to be molded or after molding, a predetermined circumferential partof the cylinder is eliminated in an axial dimension to take intoconsideration of the shrinkage after sintering. The radial crosssectional shape molding is made into the letter C and sintered, and assintered or after the out of roundness processing, the uniformcylindrical magnet is obtained except for slits, as shown in FIG. 2˜FIG.4.

In this invention, the object ferrite magnets are, as long as it has theferrite composition which includes Sr or Ba, all magnets containing theferrite composition regardless of its magnetic properties, and themanufacturing method of this invention can be applied.

That is to say, as the raw material powder, Sr ferrite pulverized powderwith less than 2 micro meter average particle size and only Ba Ferritepulverized powder can be used, and the mixture of these powders and Srferrite granulated powder which was difficult to manufacture withoutcracking using the conventional method (Patent Bulletin Heisei 1-48648)can also be used; furthermore, it is possible to use the raw materialpowder which contains other specified additives than Sr ferrite and Baferrite, and it is desirable to select according to desired moldingdimensions and magnetic properties.

In this invention, a slit means a through slit which is formedthoroughly in radial and axial directions and not a hollow. It means aslit which is made by removing a part of the circumference of thecylinder shape molding and closed after sintering, but it is not limitedto those slits that are totally closed prior to the sintering and it canhave a small space. The proposal is made actively to utilize these smallspace in the following examples.

Also, it is not limited to those which form parallel to the radialdirection or to the axial direction, but as it is shown in the actualexample later, many shapes can be applied. According to the shapes, inaddition to the effect due to a slit existence, other effects can alsobe attained.

Furthermore, the radial anisotropic cylinder shape ferrite magnet with aslit, the molding of which must be made into the letter C prior tosintering. But, this can be accomplished by first making the molding dieinto the required shape, or after it is molded into a cylindrical form,a silt with necessary width is made by the cutting and grinding work.Whichever method is chosen, these slits are determined inconsiderationof shrinkage after sintering. Since the shrinkage rate is determined bythe ferrite compositions, the molding dimension, and the sinteringcondition, they must be determined carefully with each factor. Moreover,these moldings are not only limited to the dry molding method but thesame effect can be obtained from the wet method.

The sintering condition for the radial anisotropic cylinder shapedferrite magnet used for this invented motor is selected according to theabove stated ferrite composition or the molding dimensions. It isrecommended, particularly, that the internal stress due to thedifference of shrinkage rate of circumferencial and radial direction insintering can easily released through a slit made on the molding, sothat the sintering condition and the shape of a jig must be carefullydesigned to accomplish it. Also, the sintering process alone produces arequired cylinder shape, but to make it further into a cylinder shapedferrite magnet with the specified inside and outside diameters, the outof roundness process is applied by the usual cutting and grinding work.

This invention is characterized by preventing accumulation of theinternal stress when sintering a cylinder shaped powder moldingconsisting of Sr and/or Ba ferrite, for example, by eliminating acircumferential section of the cylinder shaped molding axially along tomake the cross sectional shape into the letter C, and sintering it. Bymaking the shape of the molding into the letter C, the space in which toalleviate the internal stress due to differences in the shrinkage rateof circumferencial and radial direction is installed by a slit torelease the stress. Thus, even using the raw material which has strongmagnetic properties, cracking will not result in sintering, the highperformance radial anisotropic cylinder shape magnet can be easilyobtained, and the high performance motor can be made by effectivelypositioning the high performance radial anisotropic cylinder shapeferrite magnets.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a stator plan of an example of this invented motor.

FIG. 2 is a partially expanded plan of a slit in this invented radialanisotropic cylinder shape ferrite magnet.

FIG. 3 is a partially expanded plan of a slit in this invented radialanisotropic cylinder shape ferrite magnet.

FIG. 4 is a partially expanded plan of a slit in this invented radialanisotropic cylinder shape ferrite magnet.

FIG. 5 is a partially expanded plan which shows the connection of magnetand yoke which consists stator in this invented motor.

FIG. 6 is a plan which shows the composition to illustrate themechanical strength of magnet which comprises stator in this inventedmotor.

FIG. 7 is an another example plan of this invention, which shows onlythe rotor composition of the motor.

FIG. 8 is a plan which shows a part of stator of motor using the usualferrite magnets.

FIG. 9 is a plan which shows a part of stator of motor using the usualferrite magnets.

FIG. 10 is a plan which shows a part of rotor of the motor compositionusing the ferrite magnet.

BEST MODE FOR CARRYING OUT THE INVENTION Example 1

Using 100% pulverized powder of Sr ferrite with the average particlesize of less than 2 micro meter, in order to manufacture a cylindershaped ferrite magnet with the dimension of 50 mm (outer radius)×40 mm(inner radius)×20 mm (thickness), two groups of moldings with theidentical dimension were made. One group was sintered as usual, while acircumferential section from the other molding group was eliminated tomake a cross sectional letter C and sintered using the inventedmanufacturing method. Two types of the radial anisotropic cylinder shapeferrite magnets were manufactured.

Magnets obtained from the same molding condition (molding pressure 1ton/cm²) and the sintering condition (1200° C.×1 hour) both had themagnetic characteristics of Br=3.8 kG, _(B) H_(C) =3.05 kG, and(BH)max=3.3 MGOe.

As far as the rate of cracking, while the usual method produced crackingin all, this invented method produced none.

Example 2

Using 100% pulverized powder of Ba ferrite with the average particlesize of less than 2 micro meter, in order to manufacture a cylindershaped ferrite magnet with the dimension of 65 mm (outer radius)×50 mm(inner radius)×25 mm (thickness), two groups of moldings with theidentical dimension were made. As in Example 1, one group was sinteredas usual, while the other group utilized the invented manufacturingmethod to obtain two types of the radial anisotropic cylinder shapeferrite magnets.

Magnets obtained from the same molding condition (molding pressure 1ton/cm²) and the sintering condition (1200° C.×1 hour) both had themagnetic characteristics of Br=3.6 kG, _(B) H_(C) =2.0 kG, and(BH)max=2.7 MGOe.

As far as the rate of cracking, while the usual method produced crackingin all, this invented method produced none.

Example 3

Using the mixture of 50˜80 wt %, pulverized powder of Sr ferrite withthe average particle size of less than 2 micro meter and 50˜20 wt %granulated Ba ferrite with 14˜200 meshes, in order to 40 mm (outerradius)×32 mm (inner radius)×15 mm (thickness), two groups of moldingswith the identical dimensions were made. As in Example 1, one group wassintered as usual, while the other group utilized the inventedmanufacturing method to obtain two types of the radial anisotropiccylinder shape ferrite magnets.

Magnets obtained from the same molding condition (molding pressure 1ton/cm²) and the sintering condition 1200° C.×1 hour) both had themagnetic characteristics of Br=3.6 kG, BHC=2.95 kG, and (BH)max=2.75MGOe.

As far as the rate of cracking, while this invented method producednone, the usual method produced cracking in all magnets.

Example 4

In FIG. 1, 10 denotes ferrite magnet containing Sr which is made fromthe above manufacturing method, and except at a slit 11 it is a uniformradial anisotropic cylinder shape ferrite magnet. Here, the out ofroundness process is administered after sintering, the magnet ispressure fastened to the inner circumference 2 of the cylinder shapeyoke to comprise stator. Furthermore, by positioning rotor (not shown)in space 3 of the inner circumference of the ferrite magnet 10, theobject motor is obtained.

The slit 11 which is formed on the radial anisotropic cylinder shapeferrite magnet 10, as shown in FIG. 2 is not limited to a slit 11acomposition which is parallel to the radial or axial directions, but asshown in FIG. 3, a slit 11b (the magnet thickness gradually changes inthe axial direction) can be oblique to the axial direction, or as shownin FIG. 4, a slit 11c (the magnet thickness gradually changes in theradial direction) which is oblique to the radial direction is possible.

Particularly, in the constructions of FIG. 3 and FIG. 4, the magneticdisturbance from the existence of the slits can be moderated. In theconstruction of FIG. 3, since the mechanical strength improves againstthe axial force, when the radial anisotropic cylinder shape ferritemagnet is pressure fastened to the inner circumference of the cylindershape yoke 2, it can prevent damage.

In FIG. 1 to FIG. 4 the slits were shown magnified, in this inventedmotors whether slits are connected or disconnected as the figures thesame effect can be obtained.

Example 5

In FIG. 5 shows the same when a slit is not connected and howeffectively it is used. If the protrusion 21 to determine the magnetposition is made in the inner circumference of the cylinder shape yoke2, it is possible to align a slit 11 to the protrusion 21, which resultsin improving assembly accuracy of the radial anisotropic cylinder shapedferrite magnet 10 and the cylinder shaped yoke 2. If further improvesaccuracy of positioning other composition materials (not shown) whichare positioned using the cylinder shaped yoke 2 as the reference and theradial anisotropic cylinder shape ferrite magnet 10, and which will alsodecrease the torque disturbance generation of motors.

Also, when magnetizing the radial anisotropic cylinder shaped ferritemagnet 10 by itself before pressure fastened to the cylinder shaped yoke2, or magnetizing it after it is pressure fastened to the cylinder shapeyoke 2, as in the case of the protrusion 21 which is to help determinethe position of the magnet, by making a protrusion to determine themagnet position in the magnetizer (not shown) and aligning it with theslit 11, it is possible to increase accuracy in the magnetic polarpositioning.

Furthermore, the protrusion 21 formed on the inner circumference of theabove mentioned cylinder shape yoke 2, which is to determine theposition of magnet, can also prevent circumferential movement after theradial anisotropic cylinder shaped ferrite magnet 10 is fastened bypressure of adhesives to the cylinder shaped yoke 2.

When concerned about the mechanical strength of the radial anisotropiccylinder type ferrite magnet 10 due to the presence of a slit 11, aftersintering, adhesives and other resins can be filled into the slit 11 andhardened; or as shown in FIG. 6,a magnetic thin plate 12 with aspecified thickness can be inserted; furthermore, the outercircumference of the magnet 10 is surrounded by a magnetic thin plate13. It is possible to make handling of the radial anisotropic cylindershape ferrite magnet easy by utilizing such compositions.

In any of the above constructions, even not applying special shapes suchas in FIG. 3 and FIG. 4 to the slit 11 formed on the radial anisotropiccylinder shape magnet 10, if it is placed at the neutral position of themagnetic field made by the magnet 10 and give consideration to theresultant magnetic pole positions, the magnetic disturbance due to theslit 11 will not occur and does not interfere with motorcharacteristics.

Example 6

FIG. 7 shows other example of this invention, particularly, it onlydisplays the rotor composition of motor. That is to say in FIG. 7, themagnet 10 consists of the ferrite composition with Sr which ismanufactured according to the above method, and except at a slit 11 atthe radial and axial directions, it is a uniform radial anisotropiccylinder shape ferrite magnet. Here, the out of roundness process isapplied after sintering, and fastened to the outer circumference of themagnet support 5 to which a rotor axis 4 is fastened in the center. Theyare positioned inside stator (not shown) to comprise motor.

In this construction, too, for the radial anisotropic cylinder shapeferrite magnet 10, the same technology used for the above statorconstruction can be utilized.

Especially, a protrusion (not shown) to determine the magnet position isformed on the outer circumference of the magnet support 5, by fasteningthe slit 11 along the protrusion, the circumferential fastening strengthof the radial anisotropic cylinder shaped ferrite magnet 10 improves andprevent the movement in the rotor revolution.

Example 7

The sintered body obtained in Example 1 was further processed by theout-of-roundness method to attain the desired dimension, and pressurefastening it to the cylinder shape yoke to obtain the invented motorshown in FIG. 1.

Positioning the radial anisotropic segment ferrite magnets which isequivalent in the magnetic properties as in the above stated radialanisotropic cylinder shape ferrite magnet, and the dimension such as theoutside diameter, inside diameter, and the height with the angle θ135°as shown in FIG. 9 the usual motor was made and compared. This inventedmotor, in comparison to the conventional motor, reduced the coggingnoise by about 50˜60%, and the increase total magnetic flux resulted inthe improvement of 15˜20% torque revolution.

Example 8

The sintered body obtained in Example 2 was further processed by theout-of-roundness method to attain the desired dimension, and pressurefastening it to the cylinder shape yoke to obtain the invented motorshown in FIG. 1.

Positioning the radial anisotropic segment ferrite magnets which isequivalent in the magnetic properties as in the above stated radialanisotropic cylinder shape ferrite magnet, and the dimension such as theoutside diameter, inside diameter, and the height with the angle θ135°as shown in FIG. 9 the usual motor was made and compared. This inventedmotor, in comparison to the conventional motor, reduced the coggingnoise by about 60˜70%, and the increase total magnetic flux resulted inthe improvement of 10˜15% torque revolution.

INDUSTRIAL APPLICABILITY

This invention, as it is obvious from examples mentioned above, enablesthe manufacturing of a high performance radial anisotropic cylindershaped ferrite magnet, which hitherto could not be manufactured due tocracking in sintering. Also, the radial anisotropic cylinder shapedferrite magnet according to this invention, as compared with using theusual ferrite compositions, it the cracking rate remarkable decreaseswhile the magnet mechanical strength improves.

This invented motor, as it is obvious from examples mentioned above, bypositioning the high performance radial anisotropic cylinder shapedferrite magnet which could not be produced to due to cracking, incomparison to the high performance conventional cylinder shaped ferritemagnets which were assembled from segment magnets, its magnet assemblyis simplified and can achieve the better efficiency in the motorassembly.

Furthermore, it practically possesses the same magnetic fielddistribution as a uniform radial anisotropic cylinder shaped ferritemagnet, the cogging generation can be lowered and the noise due tocoggings can be lowered. Furthermore, in comparison to segment magnets,since the total magnetic flux generated increased, if the same power output is desired the minaturiztaion can be accomplished.

We claim:
 1. A radially anisotropic cylindrical ferrite magnet composedof a ferrite containing at least one element selected from the groupconsisting of Sr and Ba, said ferrite magnet having a complete roundnessformed by grinding the inner and outer peripheral surfaces of thecylindrical ferrite magnet after sintering thereof and a slit extendingin an axial direction thereof, and being an integrated one-piece bodyexcept for said slit.
 2. The radially anisotropic cylindrical ferritemagnet as claimed in claim 1, wherein said slit extends in parallel toboth said axial direction and a radial direction of said cylindricalferrite magnet.
 3. The radially anisotropic cylindrical ferrite magnetas claimed in claim 1, wherein said slit extends diagonally to both aradial direction and said axial direction of said cylindrical ferritemagnet.
 4. The radially anisotropic cylindrical ferrite magnet asclaimed in claim 1, wherein said slit extends diagonally to a radialdirection of said cylindrical ferrite magnet, but in parallel to saidaxial direction thereof.
 5. The radially anisotropic cylindrical ferritemagnet as claimed in claim 1, including a resin material filling saidslit.
 6. The radially anisotropic cylindrical ferrite magnet at claimedin claim 1, including a thin magnetic plate in said slit.
 7. Theradially anisotropic cylindrical ferrite magnet at claimed in claim 1,wherein said magnet is encircled with a thin magnetic plate.
 8. A motorcomprising a radially anisotropic cylindrical ferrite magnet composed ofa ferrite containing at least one element selected from the groupconsisting of Sr and Ba, said ferrite magnet having a complete roundnessformed by grinding the inner and outer peripheral surfaces of thecylindrical ferrite magnet after sintering thereof and a slit extendingin an axial direction thereof, and being an integrated, one-piece bodyexcept for said slit.
 9. The motor as claimed in claim 8, wherein acylindrical yoke is provided on its inner periphery with a protrusionwhich is fitted in said slit to fix said ferrite magnet at apredetermined position.
 10. The motor as claimed in claim 8, including aresin material filling said slit.
 11. The motor as claimed in claim 8,including a thin magnetic plate in said slit.
 12. The motor as claimedin claim 8, wherein said magnet is encircled with a thin magnetic plate.